Polarization method of a multi-layered piezoelectric body

ABSTRACT

In a polarization method of the multi-layered piezoelectric body in which a plurality of piezoelectric layers and a plurality of internal electrodes are alternately laminated and adjacent piezoelectric layers are polarized in the thickness direction such that the polarized directions thereof are in opposite directions, a first polarization process in which an electric field in one direction is applied in the thickness direction to the multi-layered piezoelectric body and a polarization is uniformly performed in the thickness direction, and a secondary polarization process in which an electric field in the opposite direction is applied to the piezoelectric layers on both sides of one of the internal electrodes and the direction of polarization of only one of the piezoelectric layers on one side of the internal electrode is reversed are provided. The secondary polarization is performed in the range such that the remaining polarization degree Pr 2  that exists after the secondary polarization in the piezoelectric layer  1   b  in which the direction of polarization is reversed does not exceed the remaining polarization degree of Pr 1  that exists after the first polarization.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polarization method of amulti-layered piezoelectric body used for a filter of a portabletelephone, or other suitable electronic component, and moreparticularly, the present invention relates to a polarization method ofa multi-layered piezoelectric body in which a plurality of piezoelectriclayers and a plurality of internal electrodes are alternately laminatedand adjacent piezoelectric layers are polarized in the thicknessdirection such that the polarization directions thereof are opposite toeach other.

[0003] 2. Description of the Related Art

[0004] Conventionally, a length mode piezoelectric resonator has largedesign freedom, small spurious vibrations, and the difference df betweena resonance frequency and an anti-resonance frequency is large. See, forexample, Unexamined Japanese Patent Publication No. 10-4330 gazette.

[0005]FIG. 1 shows an example of this length mode piezoelectricresonator 10. The piezoelectric resonator 10 includes a base 11 in whicha plurality of piezoelectric layers 12 and a plurality of internalelectrodes 13 are laminated alternately. The piezoelectric layers onboth sides of the internal electrodes 13 are polarized in oppositedirections. Insulating films 14 and 15 are alternately provided to coverends of the internal electrodes 13. Furthermore, external electrodes 16and 17 are provided on opposing surfaces of the piezoelectric resonator10. Therefore, the external electrodes 16 and 17 are alternatelyconnected to every other one of the internal electrodes 13.

[0006] In the piezoelectric resonator 10, the polarization degree of thepiezoelectric layer 12 greatly influences the properties thereof.Therefore, variations in the polarization degree within each element andvariations in the polarization degree variation between elements must beminimized.

[0007] In this type of piezoelectric resonator, a block-likemulti-layered piezoelectric body is provided. After polarization isperformed, the piezoelectric body is cut into separate piezoelectricresonators. The polarization process of the multi-layered piezoelectricbody is performed by the method shown in FIG. 2. A multi-layeredpiezoelectric body 1 is defined by a block-like piezoelectric ceramicmaterial member. Here, although four piezoelectric layers 1 a to 1 d areshown to simplify the explanation, many layers are laminated together toproduce the piezoelectric resonator. Between the piezoelectric layers 1a to 1 d, internal electrodes 2 a to 2 c are provided. The internalelectrodes 2 a to 2 c are alternately led out to a side surface of thepiezoelectric body 1, and are connected with side surface electrodes 3and 4. Also, by applying the DC electric field between the side surfaceelectrodes 3 and 4, as illustrated by arrow P, the piezoelectric layers1 b and 1 c on both sides of the internal electrode 2 b are polarized inopposite directions thereby obtaining a desired polarization degree.

[0008] However, in the method as shown in FIG. 2, because an electricfield concentrates on the edge portion of the internal electrodes 2 a to2 c, the polarization degree distribution is not uniform. FIG. 3 showsan example of the polarization degree distribution in one piezoelectriclayer. An oblique line illustrates the polarization degree. As shown inthe FIG. 3, if the electric field is applied in the thickness directionto the piezoelectric body 1, the polarization degree at the four cornersections of the piezoelectric body 1 is substantially increased (concavedistribution), and a uniform polarization degree distribution is notobtained. As a result, when lamination elements used to form the blockhaving piezoelectric layers with non-uniform polarization degreedistributions are laminated, and the lamination is cut into arectangular shape to define an element, it is impossible to use theperipheral sections of the piezoelectric body, and thus the yield of thepiezoelectric body is greatly reduced.

[0009] For example, when performing the polarization of a multi-layeredpiezoelectric body for series resonators (fr=450 kHz, df=55 kHz) usedfor a ladder-type filter by the method shown in FIG. 2, the variation inthe polarization degree df in the block is at least 10 kHz. Therefore,only elements cut from near the center of the block can be used, and thepolarization of the peripheral element of the block is defective andcannot be used.

[0010] Consequently, the inventions of the present application havesuggested a method in which an electric field is applied to opposingexternal electrodes of a main surface of a multi-layered piezoelectricbody, after performing the polarization (initial polarization) in onethickness direction of the multi-layered piezoelectric body, a sidesurface electrode which leads an internal electrode out is alternatelyprovided. An electric field is applied between the side electrodes andonly the direction of polarization of the piezoelectric layer of oneside of the internal electrode is reversed (polarization reversal), anda desired polarization degree is obtained. See, for example, JapaneseUnexamined Patent Application No. 2000-52743. In this method, as shownin FIG. 4, even when there is a variation in polarization degree ΔP1between a peripheral section and a center section in the initialpolarization, when the electric field is applied in the oppositedirection and the direction of polarization is reversed, polarizationdegree variations are reduced to ΔP2. Thus, the non-uniformity of thepolarization degree distribution after an initial polarization iscorrected.

[0011] However, when the polarization of a saturated polarization degreePmax of the piezoelectric layer having a direction of polarization thatis reversed is performed until it becomes almost equal to saturatedpolarization degree Pmax at the time of an initial polarization,although the polarization degree variation is reduced, a polarizationdegree distribution of the piezoelectric layer in which polarizationreversal is performed becomes concave in a similar manner as before thepolarization reversal. Therefore, when a multi-layered piezoelectricbody is constructed in which the piezoelectric layers are polarized in areverse direction are laminated alternately by the above-mentionedmethod, the piezoelectric layer with a concave distribution in which thepolarization reversal is performed and the piezoelectric layer with aconcave distribution in which the polarization reversal is not performedare laminated alternately. Also, in a multi-layered piezoelectric bodyas a whole, a uniform polarization degree distribution is not reliablyobtained.

SUMMARY OF THE INVENTION

[0012] To overcome the above-described problems, preferred embodimentsof the present invention provide a polarization method for amulti-layered piezoelectric body that produces a polarization degreedistribution of the entire multi-layered piezoelectric body that isuniform, thus greatly improving the yield.

[0013] According to a preferred embodiment of the present invention, apolarization method for a multi-layered piezoelectric body includesalternately laminating a plurality of piezoelectric layers and aplurality of internal electrodes and polarization of the adjacentpiezoelectric layers is performed in the thickness direction thereof soas to polarize the adjacent piezoelectric layers in opposite directions,a first polarization process in which an electric field in one thicknessdirection is applied to the multi-layered piezoelectric body anduniformly performs the polarization in the thickness direction, and asecondary polarization process in which the electric field is applied inthe opposite thickness direction to the piezoelectric layers on bothsides of the internal electrodes and only the direction of polarizationof the piezoelectric layer of one side of the internal electrode isreversed. The above secondary polarization is performed in a range suchthat a remaining polarization degree Pr2 that exists after the secondarypolarization in the piezoelectric layer having a direction ofpolarization that is reversed does not exceed a remaining polarizationdegree Pr1 that exists after the first polarization.

[0014] Further, according to a preferred embodiment of the presentinvention, in a polarization method of the multi-layered piezoelectricbody in which a plurality of piezoelectric layers and a plurality ofinternal electrodes are laminated alternately and the polarization ofthe adjacent piezoelectric layers is performed in opposite thicknessdirections, a first polarization process is performed in which electricfields of opposite directions are applied to the piezoelectric layers onboth sides of the internal electrode and the polarization is performedon the piezoelectric layers on both sides of the internal electrode inopposite directions, and a secondary polarization process is provided inwhich an electric field in the opposite direction of the electric fieldin the first polarization process is applied in opposite directions andthe direction of polarization of the piezoelectric layers of both sidesof the internal electrode is reversed, wherein the above secondarypolarization is performed in a range such that the remainingpolarization degree Pr2 that exists after the secondary polarization inthe piezoelectric layer having a direction of polarization that isreversed does not exceed the remaining polarization degree of Pr1 thatexists after the first polarization.

[0015] A first polarization is performed by applying an electric fieldin one thickness direction to a multi-layered piezoelectric body, touniformly polarize the multi-layered piezoelectric body in the thicknessdirection. Next, a secondary polarization is performed by applying anelectric field in an opposite direction relative to the piezoelectriclayers on both sides of the internal electrode and only the direction ofpolarization of the piezoelectric layer of one side of the internalelectrode is reversed.

[0016] Other features, elements, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a perspective diagram of an example of a piezoelectricresonator according to a preferred embodiment of the present invention.

[0018]FIG. 2 is a diagram showing a polarization method of theconventional multi-layered piezoelectric body.

[0019]FIG. 3 is a perspective diagram showing a polarization degreedistribution of a block-like piezoelectric body in which a polarizationwas performed by the method of FIG. 2.

[0020]FIG. 4 is a diagram showing the polarization degree distributionat a time of the initial polarization and the reversed polarization.

[0021]FIG. 5 is a process drawing showing an example of a polarizationmethod in accordance with a preferred embodiment of the presentinvention.

[0022] FIGS. 6(a) to 6(d) are diagrams showing variations in thepolarization degree distribution of two adjacent piezoelectric layerswhen performing the polarization method shown in FIG. 5.

[0023]FIG. 7 is a process drawing showing an example of a polarizationmethod according to a preferred embodiment of the present invention.

[0024] FIGS. 8(a) to 8(d) are diagrams showing variations in apolarization degree distribution of adjacent two piezoelectric layerswhen performing the polarization method shown in FIG. 7.

[0025]FIG. 9 is a diagram showing variations in the polarization degreeof the reversed polarization layer when changing the applied voltage ofa secondary polarization.

[0026]FIG. 10 is a diagram showing the polarization degree distributionin each point of A-F of FIG. 9.

[0027] FIGS. 11(a) to 11(c) are process drawings showing an example ofthe polarization method in accordance with the first example ofpreferred embodiments of the present invention.

[0028] FIGS. 12(a) to 12(d) are process drawings showing an example ofthe polarization method in accordance with the second example ofpreferred embodiments of the present invention.

[0029] FIGS. 13(a) to 13(c) are process drawings showing an example ofthe polarization method in accordance with the third example ofpreferred embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030]FIG. 5 shows an example of a polarization method according to apreferred embodiment of the present invention. First, front and backelectrodes 5 and 6 are provided on front and back surfaces of themulti-layered piezoelectric body 1, the DC electric field in thethickness direction is applied to the multi-layered piezoelectric body1, and the polarization (first polarization) is performed in onethickness direction. Then, the internal electrodes 2 ato 2 c arealternately led out to the outside surface of the multi-layeredpiezoelectric body 1 and connected to side surface electrodes 3 and 4.By applying the DC electric field between the side surface electrodes 3and 4, electric fields of opposite directions are applied to thepiezoelectric layers 1 b and 1 c on both sides of the internal electrode2 b and only the direction of polarization of the piezoelectric layer 1b of one side of the internal electrode 2 b is reversed (secondarypolarization). In addition, repolarization of only the piezoelectriclayer 1 c on the other side of the internal electrode 2 b is performed.Thus, the direction of polarization is not reversed.

[0031] The polarization conditions at the time of the secondarypolarization were changed, and the inventor discovered that apolarization degree distribution of the piezoelectric layer 1 b havingthe reversed polarization axis varies with the degree (the remanentpolarization degree) of progress of the secondary polarization.

[0032] FIGS. 6(a) to 6(d) show the changes in the degree of distributionof polarization in the two piezoelectric layers 1 b and 1 c as thesecondary polarization is performed. An arrow shows the direction of thepolarization.

[0033]FIG. 6(a) shows the polarization degree distribution after thefirst polarization. The polarization degree distribution includesconcave distributions. FIGS. 6(b) to 6(d) show a variation of thepolarization degree distribution as the secondary polarization isperformed. In addition, Pr1 shows a remaining polarization degree causedby the first polarization. Pr2 shows a remaining polarization degreecaused by the secondary polarization.

[0034] As clearly shown in FIG. 6, the polarization degree distributionof the piezoelectric layer 1 b is a convex-shaped or flat distributionsuch as (b) and (c) when the direction of polarization is initiallyreversed. However, as shown in (d), the polarization degree distributionchanges to a concave shape as the secondary polarization progresses. Inaddition, the polarization degree distribution of piezoelectric layer 1b in (b) and (c) is convex-shaped in the example shown in FIG. 6.However, some materials have an approximately flat distribution. Thus,if the secondary polarization progresses too much, the polarizationdegree distribution of the piezoelectric layer 1 b becomes a similarconcave distribution as produced by the first polarization (however, thedirection of a polarization is reversed). In addition, the polarizationdegree distribution of the piezoelectric layer 1 c by remains concave.

[0035] Therefore, as shown in (d), when the reversed polarizationprogresses too much, the polarization degree distribution of thepiezoelectric layer 1 b in which the reversal polarization is performedand a polarization degree distribution of the piezoelectric layer 1 c inwhich reversed polarization is not performed are both concave. Thus, auniform polarization degree distribution in the overall piezoelectricbody is not produced.

[0036] Consequently, the secondary polarization is terminated in a rangesuch that the polarization degree distribution of the piezoelectriclayer 1 b having a direction of polarization that is reversed is eitherconvex-shaped or flat. In other words, the secondary polarization isperformed in a range wherein the remaining polarization degree Pr2 thatexists after the secondary polarization does not exceed the remainingpolarization degree Pr1 that exists after the first polarization, suchthat Pr1≧Pr2.

[0037] When the above equation is satisfied, since the polarizationdegree distribution of the piezoelectric layer 1 b in which thedirection of polarization is reversed is convex or flat and thepolarization degree distribution of piezoelectric layer 1 c in which thedirection of polarization is not reversed is concave, the convex-shapedpolarization degree distribution and the concave polarization degreedistribution offset each other. Further, the degree of unevenness of theconcave distribution is greatly reduced. Thus, the multi-layeredpiezoelectric body achieves a substantially uniform polarization degreedistribution. As a result, the usable portion of the piezoelectric bodygreatly increases, and therefore the yield greatly improves.

[0038] In addition, where Pr1>Pr2, the amplitude of the polarizationdegree of the two layers 1 b and 1 c is unbalanced. However, when thepolarization degree distribution is substantially uniform, the resonancecharacteristic of a length vibration mode element is not adverselyaffected.

[0039] Where Pr2≈Pr1, as shown in (c) of FIG. 6, the size of thepolarization degree of the two layers 1 b and 1 c is approximatelyequal, and the distribution of the multilayer piezoelectric body isuniform, resulting in greatly improved properties.

[0040] According to a second preferred embodiment of the presentinvention, a first polarization process includes a first process toapply an electric field in a first thickness direction to amulti-layered piezoelectric body, and a second process to apply theelectric field in the thickness direction that is opposite to the firstthickness direction to the multi-layered piezoelectric body. Thedirection of polarization of the multi-layered piezoelectric bodyproduced by the first process is uniformly reversed by the secondprocess.

[0041] Thus, the first polarization is performed only once. However, thepolarization degree distribution is concave after the initialpolarization of FIG. 4. Also, the difference ΔP1 between a centersection and an edge portion is large. Consequently, multiple operationsof the first polarization are performed. If a direction of polarizationis reversed over the entire multi-layered piezoelectric body, thedifference ΔP2 between a center section and an edge portion decreaseswhen polarization reversal occurs of FIG. 4. Also, the non-uniformity ofthe degree distribution of polarization is corrected. Thus, thenon-uniformity of the polarization degree distribution of thepiezoelectric layer after the secondary polarization is corrected bycorrecting the non-uniformity of the degree distribution of polarizationin the first polarization.

[0042] In addition, the second process may be performed multiple times.

[0043] According to a third preferred embodiment of the presentinvention, a first polarization that applies an electric field inopposite directions to the piezoelectric layers on both sides of theinternal electrode to produce polarization of the piezoelectric layerson both sides of the internal electrode in opposite directions isperformed. A secondary polarization in which electric fields in theopposite directions to the above-mentioned electric fields are appliedto the piezoelectric layers on both sides of the internal electrode andthe polarization axes of the piezoelectric layers on both sides of theinternal electrode are reversed. In other words, the secondarypolarization axial direction of the piezoelectric layers on both sidesof the internal electrode is reversed relative to the first polarizationaxial direction.

[0044]FIG. 7 shows an example of the polarization method describedabove. First, the internal electrodes 2 a to 2 c are alternately led outto the outside surface of the multi-layered piezoelectric body 1. Sideelectrodes 3 and 4 are electrically connected to the internal electrodes2 a to 2 c. In addition, the polarization is performed on thepiezoelectric layers 1 b and 1 c on both sides of the internal electrode2 b in opposite directions by applying a DC electric field between theside surface electrodes 3 and 4 (first polarization). This polarizationprocess is the same as the polarization process in the prior art (FIG.2). Next, a DC electric field in a reversed direction is applied betweenthe side surface electrodes 3 and 4, and the polarization axes of thepiezoelectric layers 1 b and 1 c on both sides of the internal electrode2 b are reversed simultaneously (secondary polarization).

[0045] Also in this case, the secondary polarization is performed in arange such that the remaining polarization degree Pr2 existing after thesecondary polarization in the piezoelectric layers 1 b and 1 c havingpolarization axes that are reversed does not exceed remainingpolarization degree Pr1 existing after the first polarization.

[0046] In this case, because the polarization axes of the piezoelectriclayers 1 b and 1 c on both sides of the internal electrode 2 b arereversed and a repolarization layer is not present, the polarizationconditions of the two layers 1 b and 1 c are approximately equal. Whileattaining equalization of a polarization degree distribution, theamplitude of the polarization degree of the two layers 1 b and 1 c isalso approximately equal.

[0047]FIG. 8 shows a change of the polarization degree distribution inthe piezoelectric layers 1 b and 1 c when performing the secondarypolarization by the method shown in FIG. 7. An arrow shows the directionof the polarization.

[0048]FIG. 8(a) shows the polarization degree distribution after thefirst polarization. The polarization degree distribution is concave.FIGS. 8(b) to 8(d) show a variation of the polarization degreedistribution when the secondary polarization is performed. After thedirection of polarization is reversed by the secondary polarization, thedegree of polarization of the two layers 1 b and 1 c is approximatelyequal, and the polarization degree distribution becomes flat or convexshaped. The degree of unevenness of the degree distribution ofpolarization of both the layers 1 b and 1 c is greatly reduced. If thesecondary polarization is performed until the remaining polarizationdegree Pr2 is approximately equal to the remaining polarization degreePr1 caused by the first polarization, as shown in (c), the two layers 1b and 1 c have a flat or convex-shaped distribution, the polarizationdegree increases, and polarization degree distribution is optimized. Ifthe secondary polarization progresses further, the distribution becomesconcave as shown in (d).

[0049] Therefore, a multilayer piezoelectric body 1 having a uniformpolarization degree distribution is obtained by setting the secondarypolarization conditions such that Pr1≧Pr2.

[0050] It is desirable to perform the first polarization process on ablock-like multi-layered piezoelectric body, and to perform thesecondary polarization process to the multi-layered piezoelectric bodyhaving a substantially rectangular shape.

[0051] That is, to enhance productivity, it is preferable to performboth the first polarization and the secondary polarization on ablock-like multi-layered piezoelectric body. However, the non-uniformityof the polarization degree distribution at the time of the firstpolarization is three-dimensional, as shown in FIG. 3. Therefore, if thesecondary polarization is performed on the block-like multi-layeredpiezoelectric body, the non-uniformity of the polarization degreedistribution cannot be eliminated.

[0052] Consequently, if the secondary polarization is performed aftercutting the block-like piezoelectric body into a substantiallyrectangular shape, the electric field strength and the duration can beset depending upon the polarization degree distribution of eachsubstantially rectangular body. Therefore, variations in the degree ofpolarization between different substantially rectangular bodies and inthe substantially rectangular body is greatly reduced.

[0053] In the multi-layered piezoelectric body in which the firstpolarization is performed, a length vibration is not excited because thepolarization is uniformly performed in the thickness direction.

[0054] Consequently, a difference DF between the resonance frequency andan antiresonance frequency of an area expansion mode is calculated, andthe degree of polarization is determined. On the other hand, in themulti-layered piezoelectric body in which the secondary polarization isperformed, the length mode vibration is excited because the bodyincludes layers in which the directions of polarization are different.The difference df between the resonance frequency and the antiresonancefrequency of length mode is calculated, and then the polarization degreeis determined. Because two types of polarization degrees havingdifferent vibration modes cannot be compared, DF of an area expansionvibration of the multi-layered piezoelectric body after the firstpolarization is converted into electro-mechanical-coupling-coefficientK, and set to a remaining polarization degree Pr1. The difference df ofa length vibration of the multi-layered piezoelectric body after thesecondary polarization is converted intoelectro-mechanical-coupling-coefficient K, and set to a remainingpolarization degree Pr2. By comparing these remaining polarizationdegrees Pr1 and Pr2, the amount of the secondary polarization isdetermined.

[0055] Moreover, in the third preferred embodiment, the multi-layeredpiezoelectric body in which the first polarization is performed includesa layer having a reversed polarization direction. Therefore, a lengthvibration is excited. Thus, both the remaining polarization degree Pr1of a first polarization and the remaining polarization degree Pr2 of asecondary polarization can be calculated from the value which isattained by converting the difference df between the resonance frequencyand antiresonance frequency of a length vibration intoelectro-mechanical-coupling-coefficient K.

[0056]FIG. 9 shows variations in the polarization degree Pr2 of asubstantially rectangular layer having a direction of polarization thatis reversed when the voltage applied in the second polarization ischanged. The remaining polarization degree Pr1 is approximately equal to50 kHz in the first polarization for a desired PZT type piezoelectricceramics. FIG. 10 shows the polarization degree distribution in eachpoint A to F of FIG. 9. In addition, the change in the degree ofpolarization of the piezoelectric layer (a regular polarization layer)when changing an applied voltage is also illustrated in FIG. 9. Thethickness of a piezoelectric layer is preferably about 0.56 mm.

[0057] Clearly from FIGS. 9 and 10, in the layer in which thepolarization is reversed, depolarization is performed in accordance witha voltage increase in a secondary polarization. A direction ofpolarization is reversed when the voltage is about 900 v, and apolarization degree increases after that. If the voltage of a secondarypolarization increases to about 1000 v, after polarization reversal, theremanent polarization degree Pr2 of a polarization inversion layer willbe set to be about 50 kHz, and will become approximately equal to theremanent polarization degree of Pr1 of a first polarization. Until thevoltage of the secondary polarization increases to near 900 v to 1000 v(referring to points D and E), the polarization degree distribution isapproximately flat or slightly convex-shaped. When the voltage increasesto more than about 1000 v (referring to point F), the polarizationdegree distribution becomes concave. Therefore, the voltage of thesecondary polarization is set such that Pr1≧Pr2, in other words, thevoltage is set to be in the range of about 900 v to about 1000 v.

[0058] In addition, the polarization degree is approximately 0 in aregular polarization layer until the voltage exceeds about 500 v.However, the polarization degree increases at voltages greater thanabout 500 v. During that period, the polarization degree distributionwithin a rectangle is concave and it does not vary substantially.

[0059] In FIGS. 9 and 10, although the piezoelectric layer in which therepolarization is performed is not illustrated, in the case of arepolarization layer, the electric field of the secondary polarizationdoes not vary until the electric field in the secondary polarizationexceeds the polarization degree in the first polarization. When theelectric field in the secondary polarization exceeds the polarizationdegree of the first polarization, the polarization degree in thesecondary polarization becomes greater than the polarization degree ofthe first polarization for the first time. During that period, thepolarization degree distribution within a rectangle is still concave inthe regular polarization layer and does not vary substantially.

[0060] Various aspects of preferred embodiments and comparative examplesof the polarization method of the multi-layered piezoelectric bodyaccording to the present invention are explained below. In this example,a PZT type multi-layered piezoelectric body was used as a material of alength mode piezoelectric resonator (df=55 kHz).

FIRST EXAMPLE

[0061]FIG. 11 shows a manufacturing process of the length modepiezoelectric resonator according to a first example of preferredembodiments of the present invention.

[0062] First, an electro-conductive paste for internal electrodes thatincludes silver, palladium, an organic binder, or other suitablematerials, is applied on one side of a green sheet that includes apiezoelectric ceramics, and these sheets are alternately laminated. Thelaminated structure is integrally baked at about 1200 degree C., and themulti-layered piezoelectric body 1 having a block shape with approximatedimensions of 20 mm×30 mm×3.9 mm is formed. Front and back electrodes 5and 6 are provided on front and back surfaces of this block 1, and a DCelectric field is applied between the front and back electrodes 5 and 6,and the first polarization is performed (referring to (a) of FIG. 11).

[0063] The conditions of the first polarization are as follows: electricfield is about 1.5 kv/mm, polarization time is about 10 min, andretention-temperature is about 70 degrees C. After that, an agingprocess is performed at about 150 degrees Celsius for about 1 hour.

[0064] Next, side electrodes 3 and 4 leading out an internal electrodeare alternately provided on a side surface of the block-likemulti-layered piezoelectric body 1 after the first polarization. Then,this multi-layered piezoelectric body 1 was cut in a vertical directionto produce one substantially rectangular element using a dicer. To therectangle 1A, a DC electric field was applied to the side surfaceelectrodes 3 and 4, and the secondary polarization was performed(referring to (b) of FIG. 11). At this time, the polarization degree ofeach rectangle 1A was set to a desired value by controlling thepolarization time. The conditions of the secondary polarization are setas follows: electric-field is 1.5 kv/mm, polarization temperature is 70degrees C.

[0065] The polarization time is controlled to adjust the desiredpolarization degree (a polarization degree of the rectangle responded tothe polarization degree of a length mode element, df=55 kHz). Afterthat, the aging process was performed at about 250 degree Celsius forabout 1 hour.

[0066] The range of the secondary polarization is determined to be asdescribed in the following. The range, wherein the value in which thedifference df between the resonance frequency and the antiresonancefrequency of the length vibration mode of the rectangle 1A after thesecondary polarization is converted into anelectro-mechanical-coupling-coefficient K (remanent polarization degreePr2), does not exceed the value of the difference DF between theresonance frequency and the antiresonance frequency of the areaexpansion oscillation mode of the block 1 after the first polarizationis converted into an electro-mechanical-coupling-coefficient K (remanentpolarization degree Pr1), that is, Pr1≧Pr2.

[0067] Every other electrode of the rectangle 1A exposed at the sidesurface was coated with an insulating material after the secondarypolarization, and a silver electrode was provided thereon. This was cutby the dicer and a 1.5 mm*1.5 mm *3.8 mm length mode piezoelectricresonator 1B was obtained (referring to (c) of FIG. 11).

[0068] Since the specific structure of this piezoelectric resonator 1Bis the same as that of FIG. 1, the explanation thereof is omitted.

Comparative Example

[0069] A block-like multi-layered piezoelectric body is formed by asimilar method as the first example. Side surface electrodes foralternately leading out an internal electrode are provided on the sidesurface of the body. A DC electric field was applied to the side surfaceelectrode of the multi-layered piezoelectric body, and the polarizationwas performed (referring to FIG. 2).

[0070] Polarization conditions are as follows: electric-field is 1.5kv/mm, and retention-temperature is 70 degree C. A polarization time iscontrolled to achieve a desired polarization degree DF=2.0+/−0.2 kHz.Polarization degree DF was calculated from the difference between theresonance frequency and the antiresonance frequency of the areaexpansion oscillation mode of a block.

[0071] After that, the aging process was performed at about 250 degreeC. for about 1 hour, the piezoelectric block is cut into a desired size,and a length mode piezoelectric resonator was obtained.

[0072] The frequency characteristics of the impedance of the twoelements which obtained by the above method are measured, and the valuedf=55 kHz was measured as the difference between a resonance frequencyand an antiresonance frequency.

[0073] Table 1 and Table 2 show the comparison in the polarizationdegree df and in the resonant frequency fr between the first example andComparative Example in a property classification process, wherein σn−1is a standard deviation and r is the difference between the maximumvalue and the minimum value. TABLE 1 First Example Evaluation EvaluationEvaluation Average Lot 1 Lot 2 Lot 3 Value df Average 56.15 55.82 55.0355.67 σ_(n-1) 0.90 0.92 0.90 0.91 max. 58.5 58.5 58.5 58.50 min. 54 53.552.5 53.33 r 4.5 5 6 5.17 fr Average 450.8 449.72 449.86 450.13 σ_(n-1)0.90 1.03 1.10 1.01 max. 454.00 454.00 454.50 454.17 min. 448.00 447.00447.00 447.33 r 6.00 7.00 7.50 6.83

[0074] TABLE 2 Comparative Example Evalua- Evalua- Evalua- Evalua-Evalua- tion tion tion tion tion Average Lot 1 Lot 2 Lot 3 Lot 4 Lot 5Value df Average 56.44 56.22 56.41 56.69 56.31 56.41 σ_(n-1) 2.19 1.972.09 2.01 1.97 12.04 max. 63.00 62.50 62.50 63.00 63.00 62.80 min. 52.5051.50 52.50 53.00 53.00 52.80 r 10.50 11.00 10.00 10.00 10.00 10.30 frAverage 448.99 448.69 448.63 448.96 447.88 448.63 σ_(n-1) 1.37 1.41 1.191.29 1.15 1.28 max. 452.50 452.00 451.50 452.00 450.50 451.70 min.444.50 443.50 444.50 445.00 444.50 444.40 r 8.00 8.50 7.00 7.00 6.007.30

[0075] Tables 1 and 2 show that the average value of the standarddeviation σn−1 of df, is 2.04 kHz in the first example and 0.91 kHz inthe Comparative Example. The first example has approximately half of thevariation in a polarization degree df of the Comparative Example.Moreover, the average value of the standard deviation σn−1 of aresonance frequency fr is 1.28 kHz in the first example and 1.01 kHz inthe Comparative Example. Thus, the variation in the resonance frequencyfr has been reduced by about 30%.

SECOND EXAMPLE

[0076]FIG. 12 shows a manufacturing process of a length modepiezoelectric resonator according to a second example of preferredembodiments of the present invention.

[0077] In this example, a plurality of operations of the firstpolarization are performed in the state of a block 1 (referring to (a)and (b) of FIG. 12) and the direction of polarization was reversedthrough the entire block 1.

[0078] In this case, the difference ΔP2 between a center section and anedge portion decreases as the polarization is reversed as shown in FIG.4. Therefore, when the block is cut into a rectangle 1A in (c), thepolarization degree variation (concave distribution) between rectangles1A and within a rectangle 1A decreases. Therefore, when a secondarypolarization is performed in (c), a concave polarization degreedistribution of the piezoelectric layer 1 c is equalized.

[0079] In addition, the process of (b) is not restricted to a singleprocess, but may be performed a plurality of times.

[0080] Table 3 shows the lot variation in the polarization degree df andthe resonance frequency fr of the second example.

[0081] As shown in Table 3, the average value of the standard deviationσn−1 of a polarization degree df is 0.85 kHz and the average value ofthe standard deviation σn−1 of a resonance frequency fr is 0.96 kHz.Thus, the second example further reduces the variation. TABLE 3 SecondExample Evaluation Evaluation Evaluation Average Lot 1 Lot 2 Lot 3 Valuedf Average 53.93 53.44 54.03 53.80 σ_(n-1) 0.96 0.71 0.87 0.85 max.58.00 55.00 57.50 56.83 min. 50.00 51.50 52.00 51.17 r 8.00 3.50 5.505.67 fr Average 449.15 448.93 449.06 449.05 σ_(n-1) 0.85 0.97 1.06 0.96max. 451.50 452.00 451.50 454.17 min. 446.00 446.00 446.50 446.17 r 5.506.00 5.00 5.50

THIRD EXAMPLE

[0082]FIG. 13 shows a manufacturing process of a length modepiezoelectric resonator according to a third example of preferredembodiments of the present invention.

[0083] The electro-conductive paste for internal electrodes thatincludes silver, palladium, an organic binder, or other suitablematerial, is applied on one side of a green sheet including apiezoelectric ceramic. These are alternately laminated and areintegrally baked at about 1200 degree C., and then the multi-layeredpiezoelectric body 1 having approximate dimensions of 20 mm×30 mm×3.9 mmin a block form was produced. In addition, the side surface electrodes 3and 4 for leading out an internal electrode are alternately provided ona side surface of the multi-layered piezoelectric body 1. A DC electricfield was applied between the side surface electrodes 3 and 4 and afirst polarization was performed (referring to (a) of FIG. 13).

[0084] The conditions of the first polarization are as follows:electric-field is 1.5 kv/mm, polarization time is 10 min., andretention-temperature is about 70 degree C. After that, an aging processwas performed at about 150 degree C. for 1 hour.

[0085] Next, the block-like multi-layered piezoelectric body 1 was cutin a vertical direction using a dicer to produce a substantiallyrectangular shape. The DC electric field is applied to the cut rectangle1A via the side surface electrodes 3 and 4, and a secondary polarizationwas performed (referring to (b) of FIG. 13). At this time, the directionof the voltage application to the rectangle 1A was in the reverseddirection to that of the first polarization. The degree of polarizationof each rectangle 1A was uniformly set to a desired value by the controlof the polarization time.

[0086] The conditions of the secondary polarization are set as follows:electric-field is 1.5 kv/mm, and polarization temperature is about 70degree C. A polarization time is controlled to achieve the desiredpolarization degree (polarization degree of the rectangle responded inpolarization degree df=55 kHz of a length mode element). After that, theaging process was performed at about 250 degree C. for about 1 hour.

[0087] The range of the secondary polarization is determined as follows:A range in which a remanent polarization degree Pr2 wherein thedifference df between the resonance frequency and the antiresonancefrequency of the length oscillation mode of the rectangle after asecondary polarization is converted intoelectro-mechanical-coupling-coefficient K does not exceed a remanentpolarization degree Pr1 wherein the difference DF between the resonancefrequency and the antiresonant frequency of the length oscillation modeof the block after the first polarization is converted intoelectro-mechanical-coupling-coefficient K, that is, Pr1≧Pr2.

[0088] Every other electrode exposed at a side surface of the rectangle1A is coated with an insulating material after the secondarypolarization, and a silver electrode was provided thereon. The dicercuts this, and a 1.5 mm×1.5 mm ×3.8 mm length mode piezoelectricresonator 1B was obtained. The structure of this piezoelectric resonator1B is the same as that of the piezoelectric resonator of the firstexample.

[0089] The frequency characteristic of the impedance in piezoelectricresonator of the third example and in the piezoelectric resonator ofComparative Example is measured. df=55 kHz value was obtained as adifference between a resonance frequency and an antiresonance frequency.

[0090] Table 4 shows lot variations of the polarization degree df andthe resonance frequency fr in the third example.

[0091] As shown in Table 4, the average value of the standard deviationσn−1 of a polarization degree df is 1.03 kHz and the average value ofthe standard deviation σn−1 of a resonance frequency fr is 0.92 kHz. Thevariation in a polarization degree df is substantially greater that inthe first example. However, the variation in a resonance frequency fr isthe smallest of the three examples. TABLE 4 Third Example EvaluationEvaluation Evaluation Average Lot 1 Lot 2 Lot 3 Value df Average 54.7254.91 55.91 55.18 σ_(n-1) 0.96 0.99 1.14 1.03 max. 56.53 55.82 58.1556.83 min. 51.61 50.44 52.50 51.52 r 4.91 5.38 5.65 5.31 fr Average450.62 449.56 452.26 450.81 σ_(n-1) 0.71 0.93 1.12 0.92 max. 451.90450.98 454.00 452.29 min. 447.93 446.20 448.68 447.60 r 3.97 4.78 5.324.69

[0092] The polarization method of this invention is not limited to theabove-described examples of preferred embodiments.

[0093] For instance, the secondary polarization was performed only onceto the piezoelectric body 1A of a rectangular shape in FIGS. 11 to 13.However, the secondary polarization may be repeated several times. Inother words, the direction of an electric field may be reversed andreversal of a direction of polarization may be repeated several times.

[0094] Moreover, when the mass-production property and a polarizationdegree distribution are considered, a first polarization is performed toa block-like multi-layered piezoelectric body and a secondarypolarization is performed to the multi-layered piezoelectric body havinga substantially rectangular shape. However, a first polarization and asecondary polarization may be performed on a block-like multi-layeredpiezoelectric body, and a first polarization and a secondarypolarization may be performed to the multi-layered piezoelectric bodyhaving a substantially rectangular shape.

[0095] In addition, in the first and second examples (FIGS. 5, 6, 11 and12), after a secondary polarization, the remaining polarization degreeof the piezoelectric layer having a direction of polarization that isreversed was temporarily set to Pr2 and was explained. This is intendedto simplify the understanding of these examples. In actuality, theentire (the piezoelectric layer in which the repolarization layer andthe direction of polarization are reversed is included) rectangle aftera secondary polarization is excited by the length oscillation mode andthe remanent polarization degree thereof is set to Pr2.

[0096] As described above, according to a preferred embodiment of thepresent invention, after performing the first polarization whichuniformly polarizes the multi-layered piezoelectric body in thethickness direction, when the secondary polarization which reverses thedirection of polarization of only the piezoelectric layer of one side ofan internal electrode is performed, because the secondary polarizationis performed in a range in which the remanent polarization degree Pr2after the secondary polarization in the piezoelectric layer having adirection of polarization that is reversed does not exceed the remainingpolarization degree Pr1 after the first polarization, the polarizationdegree distribution of the piezoelectric layer having a direction ofpolarization that is reversed is convex-shaped or flat. Therefore, sincethe polarization degree distribution of the piezoelectric layer having adirection of polarization that is reversed is convex-shaped or flat, anda polarization degree distribution of the piezoelectric layer having adirection of polarization that is not reversed is concave, anapproximately uniform polarization degree distribution is obtained forthe overall multi-layered piezoelectric body. As a result, when cuttingand using a multi-layered piezoelectric body, that usable portiongreatly increases, and thus the yield greatly improves.

[0097] According to another preferred embodiment of the presentinvention, after performing the first polarization which polarizes thepiezoelectric layers on both sides of the internal electrode in oppositedirections, when the secondary polarization which reverses the directionof polarization of the piezoelectric layers on both sides of theinternal electrode is performed, because a secondary polarization isperformed in a range in which the remanent polarization degree Pr2 afterthe secondary polarization in the piezoelectric layer having apolarization axis that is reversed does not exceed the remainingpolarization degree of Pr1 after the first polarization, thepolarization of the piezoelectric layers on both sides of the internalelectrode in which the polarization is reversed. The polarization degreedistribution is substantially equalized. Also, because the polarizationdegree of the adjacent piezoelectric layers is approximately equal,outstanding resonance characteristics are achieved.

[0098] While preferred embodiments of the invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. A polarization method of a multi-layeredpiezoelectric body in which a plurality of piezoelectric layers and aplurality of internal electrodes are alternately laminated and adjacentpiezoelectric layers are polarized in the thickness direction thereofsuch that the polarization directions thereof are in oppositedirections; comprising steps of: a first polarization process in whichan electric field in a first thickness direction is applied to themulti-layered piezoelectric body and the polarization is uniformlyperformed in the first thickness direction, a secondary polarizationprocess in which an electric field in a second thickness direction thatis opposite to the first thickness direction is applied to thepiezoelectric layers of both sides of the internal electrodes and thedirection of polarization of the piezoelectric layer on only one side ofthe internal electrode is reversed; wherein the secondary polarizationprocess is performed in a range such that a remaining polarizationdegree Pr2 that exists after the secondary polarization process in thepiezoelectric layer having a direction of polarization that is reverseddoes not exceed a remaining polarization degree Pr1 after the firstpolarization.
 2. A polarization method of a multi-layered piezoelectricbody according to claim 1, wherein the above first polarization processcomprises: a first process in which an electric field in the firstthickness direction is applied to the multi-layered piezoelectric body,and a second process in which an electric field in the second thicknessis applied to the multi-layered piezoelectric body, and wherein thedirection of polarization of the multi-layered piezoelectric bodyproduced by the first process is uniformly reversed by the secondprocess.
 3. A polarization method of the multi-layered piezoelectricbody according to claim 1, wherein the first polarization is performedon a block-like multi-layered piezoelectric body and the secondarypolarization process is performed on a substantially rectangular-shapedmulti-layered piezoelectric body produced by cutting the block-likemulti-layered piezoelectric body in a vertical direction substantiallyparallel to the internal electrodes.
 4. A polarization method of themulti-layered piezoelectric body according to claim 1, wherein theelectric fields applied to the multi-layered piezoelectric body are DCelectric fields.
 5. A polarization method of the multi-layeredpiezoelectric body according to claim 1, wherein the polarization degreedistribution produced by the first polarization process has a concaveshape, and the polarization degree distribution produced by the secondpolarization process has a flat or convex shape, such that the overallpolarization degree distribution of the multi-layered piezoelectric bodyis uniform.
 6. A polarization method of the multi-layered piezoelectricbody according to claim 1, wherein the remaining polarization degree Pr2and the remaining polarization degree Pr1 are approximately equal.
 7. Apolarization method of the multi-layered piezoelectric body according toclaim 1, wherein the remaining polarization degree Pr2 is about 50 kHz.8. A polarization method of the multi-layered piezoelectric bodyaccording to claim 6, wherein the remaining polarization degree Pr1 andthe remaining polarization degree Pr2 are about 50 kHz.
 9. Apolarization method of the multi-layered piezoelectric body according toclaim 1, wherein the voltage of the electric field applied in the secondpolarization process is in the range of about 900 v to about 1000 v. 10.A polarization method of the multi-layered piezoelectric body accordingto claim 1, wherein the multi-layered piezoelectric body is made ofPZT-type piezoelectric ceramic.
 11. A polarization method of amulti-layered piezoelectric body in which a plurality of piezoelectriclayers and a plurality of internal electrodes are alternately laminatedand a polarization is performed on adjacent piezoelectric layers in thethickness direction such that the polarization directions thereof are inopposite directions, comprising; a first polarization process in whichelectric fields in opposite directions are applied to the piezoelectriclayers on both sides of the internal electrodes such that thepiezoelectric layers on both sides of internal electrodes are polarizedin opposite directions, and a secondary polarization process in whichelectric fields in the opposite directions to the electric fields in thefirst polarization process are applied to the piezoelectric layers ofboth sides of the internal electrode such that the polarization axes ofthe piezoelectric layers of both sides of the internal electrode arereversed, wherein the secondary polarization process is performed in arange such that a remaining polarization degree Pr2 that exists afterthe secondary polarization in the piezoelectric layers having adirection of polarization that is reversed does not exceed the remainingpolarization degree Pr1 that exists after the first polarization.
 12. Apolarization method of the multi-layered piezoelectric body according toclaim 11, wherein the first polarization is performed on a block-likemulti-layered piezoelectric body and the secondary polarization processis performed on a substantially rectangular-shaped multi-layeredpiezoelectric body produced by cutting the block-like multi-layeredpiezoelectric body in a vertical direction substantially parallel to theinternal electrodes.
 13. A polarization method of the multi-layeredpiezoelectric body according to claim 11, wherein the electric fieldsapplied to the multi-layered piezoelectric body are DC electric fields.14. A polarization method of the multi-layered piezoelectric bodyaccording to claim 11, wherein the polarization degree distributionproduced by the first polarization process has a concave shape, and thepolarization degree distribution produced by the second polarizationprocess has a flat or convex shape, such that the overall polarizationdegree distribution of the multi-layered piezoelectric body is uniform.15. A polarization method of the multi-layered piezoelectric bodyaccording to claim 11, wherein the remaining polarization degree Pr2 andthe remaining polarization degree Pr1 are approximately equal.
 16. Apolarization method of the multi-layered piezoelectric body according toclaim 11, wherein the remaining polarization degree Pr2 is about 50 kHz.17. A polarization method of the multi-layered piezoelectric bodyaccording to claim 15, wherein the remaining polarization degree Pr1 andthe remaining polarization degree Pr2 are about 50 kHz.
 18. Apolarization method of the multi-layered piezoelectric body according toclaim 11, wherein the voltage of the electric field applied in thesecond polarization process is in the range of about 900 v to about 1000v.
 19. A polarization method of the multi-layered piezoelectric bodyaccording to claim 11, wherein the multi-layered piezoelectric body ismade of PZT-type piezoelectric ceramic.